[DTrace-devel] [PATCH 01/10] Reduce register pressure in substr()

[Kris Van Hees]

Have substr() return the result string pointer.  This means that we can
delay allocating the register to hold the result until after the call to
dt_substr() has been made.

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 bpf/substr.S      |  3 ++-
 libdtrace/dt_cg.c | 29 ++++++++++++++++-------------
 2 files changed, 18 insertions(+), 14 deletions(-)

diff --git a/bpf/substr.S b/bpf/substr.S
index 8d501faa..9bd2400d 100644
--- a/bpf/substr.S
+++ b/bpf/substr.S
@@ -6,7 +6,7 @@
 #define BPF_FUNC_probe_read_str	45
 
 /*
- * void dt_substr(char *dst, const char *src, int32_t idx, int32_t cnt,
+ * char *dt_substr(char *dst, const char *src, int32_t idx, int32_t cnt,
  *		  uint64_t argc)
  *
  * %r1 = dst, %r2 = src, %r3 = idx, %r4 = cnt, %r5 = argc
@@ -124,6 +124,7 @@ dt_substr :
 					 *		       &src[idx]);
 					 */
 
+	mov	%r0, %r9
 	exit
 
 .Lempty:
diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
index 6f95260c..2cf48994 100644
--- a/libdtrace/dt_cg.c
+++ b/libdtrace/dt_cg.c
@@ -3997,26 +3997,20 @@ dt_cg_subr_substr(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
 		dt_cg_node(cnt, dlp, drp);
 
 	/*
-	 * Allocate the result register and associate it with a temporary
-	 * string slot.
+	 * Allocate a temporary string slot for the result.
 	 */
-	dnp->dn_reg = dt_regset_alloc(drp);
-	if (dnp->dn_reg == -1)
-		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
 	dt_cg_tstring_alloc(yypcb, dnp);
 
-        emit(dlp,  BPF_LOAD(BPF_DW, dnp->dn_reg, BPF_REG_FP, DT_STK_DCTX));
-        emit(dlp,  BPF_LOAD(BPF_DW, dnp->dn_reg, dnp->dn_reg, DCTX_MEM));
-        emit(dlp,  BPF_ALU64_IMM(BPF_ADD, dnp->dn_reg, dnp->dn_tstring->dn_value));
-
 	if (dt_regset_xalloc_args(drp) == -1)
 		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
 
-	emit(dlp, BPF_MOV_REG(BPF_REG_1, dnp->dn_reg));
-	emit(dlp, BPF_MOV_REG(BPF_REG_2, str->dn_reg));
+        emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_1, BPF_REG_FP, DT_STK_DCTX));
+        emit(dlp,  BPF_LOAD(BPF_DW, BPF_REG_1, BPF_REG_1, DCTX_MEM));
+        emit(dlp,  BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, dnp->dn_tstring->dn_value));
+	emit(dlp,  BPF_MOV_REG(BPF_REG_2, str->dn_reg));
 	dt_regset_free(drp, str->dn_reg);
 	dt_cg_tstring_free(yypcb, str);
-	emit(dlp, BPF_MOV_REG(BPF_REG_3, idx->dn_reg));
+	emit(dlp,  BPF_MOV_REG(BPF_REG_3, idx->dn_reg));
 	dt_regset_free(drp, idx->dn_reg);
 	if (cnt != NULL) {
 		emit(dlp, BPF_MOV_REG(BPF_REG_4, cnt->dn_reg));
@@ -4030,8 +4024,17 @@ dt_cg_subr_substr(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
 	idp = dt_dlib_get_func(yypcb->pcb_hdl, "dt_substr");
 	assert(idp != NULL);
 	emite(dlp,  BPF_CALL_FUNC(idp->di_id), idp);
-	dt_regset_free_args(drp);
+
+	/*
+	 * Allocate the result register, and assign the result to it..
+	 */
+	dnp->dn_reg = dt_regset_alloc(drp);
+	if (dnp->dn_reg == -1)
+		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
+
+	emit(dlp, BPF_MOV_REG(dnp->dn_reg, BPF_REG_0));
 	dt_regset_free(drp, BPF_REG_0);
+	dt_regset_free_args(drp);
 
 	TRACE_REGSET("    subr-substr:End  ");
 }
-- 
2.34.1